Process for the preparation of TiCl4

ABSTRACT

An improved process for the preparation of TiCl 4  comprising 
     (a) chlorinating a raw material containing titanium and vanadium impurities to produce a crude TiCl 4  reaction product, 
     (b) separating the crude reaction product into solid by-products and TiCl 4  -containing liquid, 
     (c) reacting the TiCl 4  -containing liquid with a reducing agent whereby vanadium impurities are converted into solid compounds, and 
     (d) separating TiCl 4  from the solid vanadium compounds, is improved by adding solid products of the vanadium reduction reactions to the crude TiCl 4  reaction product prior to the separating step (b).

This invention relates to a process for the preparation of TiCl₄ by the chlorination of raw materials containing titanium in the presence of reducing agents, separation of solid metal chlorides by cooling of the reaction gases and subsequent condensaton of the crude TiCl₄ and reduction of vanadium compounds present in the crude TiCl₄ with the formation of solid reaction products, followed by distillation.

BACKGROUND OF THE INVENTION

Titanium dioxide pigments are nowadays prepared not only by the sulphate process but also by the combustion process in which TiCl₄ and oxygen are directly converted into titanium dioxide pigments by heating to an elevated temperature.

The TiCl₄ required for this purpose is obtained by the chlorination of materials containing titanium, such as ilmenite, leucoxene or rutile, in the presence of carbon. The crude TiCl₄ obtained is contaminated with numerous other chlorides and with chlorine, the main impurities being the chlorides of iron, aluminium and silicon as well as the chlorides and oxychlorides of vanadium.

Removal of these impurities is essential if the TiO₂ pigments are to have a pure white color.

Most of the impurities, such as the chlorides of iron, aluminium and silicon, may be removed by distallation. The distillation of crude TiCl₄ results in the formation of thickened suspensions of the solid metal chlorides and of extremely finely divided residues of crude chlorination products which are difficult to evaporate to dryness, especially if they contain a high proportion of AlCl₃. This distillation therefore entails a high energy consumption and/or losses in TiCl₄ yield.

It would therefore be preferable to separate the solid metal chlorides and the finely divided residues of the solid raw material of the chlorination process by filtration and to dry the filter cake. This method, however, becomes very expensive due to the difficulty of filtering the finely divided solids.

The product obtained after removal of the solid impurities still contains vanadium in the form of VOCl₃ or VCl₄.

The removal of VOCl₃ and VCl₄ from the titanium tetrachloride by distillation is costly due to the similarity of their boiling points.

These compounds are therefore converted into solid, low valency vanadium chlorides by reduction.

A product containing only low vanadium concentrations (so-called pure TiCl₄) may then be obtained by distillation.

Known methods of purifications use, for example, H₂ S (DE-A No. 1 923 479), animal and vegetable oils, fats, waxes, resins and soaps, liquid or gaseous hydrocarbons, oils, fats, alcohols, ketones, organic acids, amines (CH-A No. 365 393, CH-A No. 262 267, DE-C No. 867 544, FR-A No. 1 466 478, FR-A No. 1 460 362), metals and metal salts (BE-A No. 539 078, DE-A No. 1 922 420, DE-B No. 1 271 693, U.S. Pat. Nos. 3,915,364, 2,871,094, 2,753,255, 2,560,424, 2,555,361, 2,530,735 and 2,178,685).

The purification of titanium tetrachloride with special cyclic aliphatic or aromatic compounds (DE-C No. 2 329 045) and with special amines (DE-C No. 2 325 924) is particularly advantageous.

It was an object of the present invention to provide a particularly advantageous process for the purification of TiCl₄ which would not have the disadvantages described above.

BRIEF DESCRIPTION OF THE INVENTION

It has surprisingly been found that the removal of the solid substances from crude TiCl₄ by filtration may be greatly facilitated by using the solid reaction products of the reduction of vanadium compounds as filtering aids for the filtration of crude TiCl₄.

The solid reaction products are mainly the solid, low valency vanadium compounds, oxidized reducing agent and possibly residues of unreacted reducing agent.

DETAILED DESCRIPTION

The present invention thus relates to a process for the preparation of TiCl₄ by the chlorination of starting materials containing titanium and vanadium as an impurity, separation of the solid metal chlorides by cooling of the reaction gases followed by condensation of the crude TiCl₄, and reduction of the vanadium compounds present in the crude TiCl₄ to form solid reduction reaction products, followed by distillation, characterised in that the solid reduction reaction products are used as filtering aids for the filtration of the crude TiCl₄.

In one embodiment of the process according to the invention, the solid reduction reaction products used are substances which were separated from TiCl₄ by filtration before distillation.

In another embodiment of the process, the solid reduction reaction products used are the substances which are obtained as distilland sump during or after distillation.

According to yet another embodiment of the process, the solid reduction reaction products used are substances which are separated by filtration of the distillation sump.

When filtration of the crude TiCl₄ is carried out, the solid reduction reaction products are added to the crude TiCl₄ and/or used as precoat layer.

Reduction of the vanadium compounds is preferably carried out with organic hydrocarbons or organic amines.

Preferred embodiment of this type are anthracene, aniline or diphenylamine.

According to one particularly preferred embodiment of the process, the organic amine used is diphenylamine and 2.5 to 5 kg of diphenylamine are used per kg of vanadium.

Reduction of the vanadium compounds with organic hydrocarbons or organic amines is preferably carried out at 80° to 125° C.

In another preferred embodiment of the process, crude TiCl₄ containing chlorine and solid substances is dechlorinated before filtration because the presence of elementary chlorine increases the quantity of reducing agent used.

BRIEF DESCRIPTION OF THE DRAWING

The process according to the invention will now be explained in more detail with reference to FIGS. 1 to 3.

FIG. 1 illustrates the process of this invention in a block diagram flowchart.

FIG. 2 illustrates second embodiment of this invention by a flowchart block diagram.

FIG. 3 illustrates a third embodiment of this invention by a block diagram flowchart. Like-numbered apparatus elements in the FIGS. are the same.

DETAILED DESCRIPTION OF THE DRAWING

In FIG. 1, crude TiCl₄ (1) containing chlorine and solids is first dechlorinated to prevent excessive consumption of reducing component.

The removal of chlorine (2) may be carried out in a dechlorination column or by heating in a stirrer vessel to 100°-130° C., the chlorine (3) released from the TiCl₄ being displaced from the vessel by nitrogen.

The dechlorinated crude TiCl₄ (4) is mixed (5) with the filter cake II (22) which is the solid reaction products of reduction of the vanadium compounds and TiCl₄ adhering thereto. The resulting mixture (6) is filtered, preferably at 50°-100° C. (7), the solids components of the filter cake II (22) acting as filtration aids which loosen up the filter cake I (8) and thereby substantially increase the efficiency of filtration.

The filtrate I (14) now contains mainly VOCl₃, VCl₄ and SiCl₄ as impurities. The vanadium compounds are reduced by the addition of reducing agent (16) to the filtrate I (14). The vanadium compounds are reduced in filtrate I (14) as described in DE-C Nos. 2 325 924 or 2 329 045 preferably at temperatures in the range of from 80° to 125° C. TiCl₄ (together with SiCl₄) is removed as pure TiCl₄ (19) from the resulting suspension (17) by distillation (18). The apparatus used for distillation is preferably a heated stirrer vessel, a horizontal evaporator or a forced circulation evaporator. The SiCl₄ is optionally removed from the TiCl₄ in a column connected in series with the distillation apparatus.

The sump (20) formed in the process of distillation (18) is filtered (21), preferably at 70° to 100° C. The filtrate II (23) obtained from this filtration II (21) is added to the filtrate I (14) either after or, preferably before reduction of the vanadium compounds (15).

The filter cake II (22) is discharged into the mixer (5) as described above to serve as filtration aid for the filtration I (7).

The filter cake II (22) may alternatively be used as precoat layer for the filtration I (7), in which case the filter is not purified after filtration II (21) but subjected to the dechlorinated crude TiCl₄ (4).

Part of the filter cake II (22) may be added to the crude TiCl₄ (4) as filtering aid while the remainder may be used as precoat layer.

The filter cake I (8) from filtration (7) contains all the solid substances resulting from the purification of the crude TiCl₄. Drying (9) of the filter cake (8) may be carried out in driers with heating surfaces heated indirectly at the ambient pressure or at higher or lower pressures or it may be carried out directly on the filter with inert gas heated to 100°-120° C., preferably nitrogen. The TiCl₄ vapor (11) formed in the process is condensed (12) and the condensate (13) is added to the crude TiCl₄ (1,4) before or, preferably, after dechlorination (2). After this initial drying, the filter cake may be further dried by blowing with an inert gas, preferably nitrogen, preferably at a temperature of 50°-110° C., before it is discharged from the filter. The resulting mixture of inert gas and TiCl₄ vapor may be transferred to the stage of dechlorination of crude TiCl₄ (2) for expelling the chlorine, either immediately or after condensation of the major proportion of its TiCl₄. The dried solids (10) are then removed from the process.

FIG. 2 illustrates a variation of the process according to the invention in which the filter cake II (27) is used as precoat layer for filtration I (24). The filter is therefore not cleaned after filtration II (26) but charged with dechlorinated crude TiCl₄ (4).

A filter cake (25) composed of two layers is obtained. This filter cake (25) also contains all the substances resulting from the purification of TiCl₄ and is dried by a method analogous to that of FIG. 1.

The vanadium compounds (15) in filtrate I (14) are then reduced as in FIG. 1. The procedure differs from that of FIG. 1, however, in that filtration II (26) is carried out immediately after reduction of the vanadium compounds (15). Distillation of TiCl₄ (29) is carried out on a filtrate II (28) which is free from solids. Circulation evaporators or horizontal evaporators are suitable for this purpose, optionally with a column arranged in series with the evaporator to enable the SiCl₄ to be removed from the pure TiCl₄ (19). The sump (30) of the distillation (29) is returned to the suspension (17).

The filter cake II (27) from filtration II (26) may alternatively also be mixed with the dechlorinated crude TiCl₄ (4) as in FIG. 1, in which case the solid components of filter cake II (27) used as filtering aids improve the efficiency of filtration I (24).

If desired, a proportion of filter cake II (27) may be added to the crude TiCl₄ while another portion is used as precoat layer.

FIG. 3 illustrates a variation of the process according to the invention analogous to that of FIG. 1. In this case, the sump (20) obtained from the distillation of TiCl₄ (18) is not filtered but directly added (5) to the dechlorinated, crude TiCl₄ (4). From 5 to 25% of the volume (17) fed into the TiCl₄ distillation (18) is normally removed as sump (20) and mixed with the dechlorinated, crude TiCl₄ (4). The disadvantage of having a larger quantity of TiCl₄ to filter is normally compensated for by the advantage that only one filtration is required for the whole process.

The filters used are pressure filters, preferably of the type of leaf filters or cartridge filters, from which the filter cake may be discharged intermittently.

The process according to the invention affords great advantages, especially for processing raw materials with only a low titanium content, such as titanium slag, synthetic rutile, leucoxene, ilmenite or Brazilian anatas, because separation of the large amounts of solid chlorides from crude TiCl₄ by the distillation of TiCl₄ is particularly difficult and direct filtration of crude TiCl₄ can in these cases only be carried out with a very low filtration output.

The advantages of the process according to the invention will now be illustrated with the aid of Examples.

EXAMPLE 1 (CORRESPONDING TO FIG. 1)

Crude TiCl₄ obtained from the chlorination of rutile sand contained 1.5% by weight of solids, 1050 ppm of vanadium in the form of VOCl₃ or VCl₄ and 580 ppm of chlorine. The crude TiCl₄ (1) was heated to 125° C. in a stirrer vessel (2) with TiCl₄ condenser attached. Nitrogen containing TiCl₄ obtained from blow-drying the filter cake I (8) was blown into the gas space of the container for 20 minutes. The chlorine-nitrogen mixture resulting from the condensation of TiCl₄ was transferred to an exhaust gas scrubber.

After this treatment, the dechlorinated crude TiCl₄ (4) contained less than 10 ppm of chlorine. The filter cake II (22) consisting of the products of reduction of the vanadium was then introduced into the stirrer vessel and mixed (5) with the crude TiCl₄ (4). The mixture (6) was cooled to 80° C. and filtered with a Fundabac® cartridge filter (DrM, Switzerland) at 5 bar (abs.) (7). The filtration output amounted on average to 2.8 m³ of filtrate/h.m² of filter surface when the filter cake had reached a thickness of 25 mm. (When, for comparison, the filtration of crude TiCl₄ (1) was carried out without the addition of filter cake II (22), the filtration output was only 0.35 m³ /h.m² when the cake had reached a thickness of 18 mm).

When filtration (7) had been completed, the turbid liquid was discharged and the filter cake was blown dry with nitrogen at 100° C. for 20 minutes. The TiCl₄ -containing nitrogen leaving the filter was used for the dechlorination (2) of the next batch of crude TiCl₄. The remaining TiCl₄ was evaporated (11) from the crumbly filter cake I (8) in an indirectly heated drier (9) and fed into the crude TiCl₄ (1) after condensation (12). The dry solids (10) were removed for treatment of the residues.

The filtrate (14) obtained from filtration I (7) was collected in an interim container and heated batchwise to 115° C. in a stirrer vessel (15), and 4 kg of diphenylamine per m³ were added (16). After 5 minutes, the resulting supsension (17) was discharged into a container from which it was continuously fed into a stirrer vessel with heating jacket (18) from which pure TiCl₄ (19) was evaporated off and finally condensed. A proportion of the suspension concentrated in the evaporator (18) was left to overflow as sump (20) and then filtered through a Fundabac® cartridge filter (21) after it had cooled to about 80° C. The filtrate (23) was added to the suspension (17) before the evaporation of TiCl₄ (18). The filter cake (22) was suspended in the dechlorinated, crude TiCl₄ (4) as described above, where it served as filtration aid for the filtration I (7). The filtration output (23) was about 3.2 m³ of filtrate/h.m² of filter surface at a pressure of 4 bar (abs.) when the filter cake (22) had reached a thickness of about 25 mm.

The pure TiCl₄ (19) was colorless. It contained less then 2 ppm of vanadium and 28 ppm of SiCl₄.

EXAMPLE 2 (CORRESPONDING TO FIG. 2)

Crude TiCl₄ (1) containing 2.9% by weight of solids with 640 ppm of V and 590 ppm of chlorine was obtained from the chlorination of Brazilian anatase. The crude TiCl₄ was dechlorinated (2) as in Example 1. The dechlorinated, crude TiCl₄ (4) was cooled to about 70° C. and filtered (24) through a Fundabac® cartridge filter at 5 bar (abs.). The cartridges of this filter were already covered with a layer of filter cake II (27) about 10 mm in thickness as precoat layer. When the filter cake reached a total thickness of about 15 mm, the filtration output in filtration I (24) was 0.8 m³ /h.m² of filter surface. (By comparison, when direct filtration of crude TiCl₄ (1) was carried out, the average filtration output obtained with a filter cake not more than 0.6 mm in thickness was 0.3 m³ /h.m². When filter cake II (27) was added to the dechlorinated, crude TiCl₄ (4) as in FIG. 1, an averge filtration output of 0.9 m³ /h.m² was obtained when the filter cake had a thickess of at the most about 15 mm.

The filter cake (25) was dried by a method analogous to that of Example 1.

The filtrate (14) was heated batchwise to 125° C. and 2.7 kg of diphenylamine per m³ (16) were added (15). After cooling to about 75° C., the suspension (17) was filtered through a Fundabac® cartridge filter at 3.5 bar (abs.) (26). The average filtration output was 6 m³ / h.m² until the maximum thickness of filtration cake of about 10 mm was reached. After discharge of the turbid liquor, the filter was directly supplied with dechlorinated, crude TiCl₄ (4) as described above (24).

Pure TiCl₄ (19) was distilled from the filtrate (28) of filtration II (26) in a horizontal evaporator wth condenser (29) attached. At the end of the horizontal evaporator remote from the inlet for filtrate, 5 to 10% of the quantity fed in (30) was withdrawn as sump and returned to the suspension (17). The pure TiCl₄ was colorless and contained less than 2 ppm of vanadium.

EXAMPLE 3 (CORRESPONDING TO FIG. 3)

Dechlorinated crude TiCl₄ (4) (as in Example 2) was mixed with the sump (20) of the TiCl₄ distillation (18) in a stirrer vessel (5). Filtration (7) and treatment of filter cake were carried out as in Example 1. The filtration output was 1.1 m³ /h.m² of filter surface at a maximum filter case thickness of 15 mm. The filtrate (14) was heated batchwise to 125° C. and 2.4 kg of diphenylamine per m³ (16) were added (15). Pure TiCl₄ (19) was then distilled from the suspension (17) containing the reaction products in a stirrer vessel (18). About 10% of the volume (17) fed in were removed in the evaporator (18) as sump (20) containing solid components and mixed with the dechlorinated, crude TiCl₄ (5).

The pure TiCl₄ was colorless and contained less than 2 ppm of vanadium. 

What is claimed is:
 1. An improved process for the preparation of TiCl₄ comprising(a) chlorinating a raw material containing titanium and vanadium impurities to produce a crude TiCl₄ reaction product, (b) separating the crude reaction product into solid by-products and TiCl₄ -containing liquid, (c) reacting the TiCl₄ -containing liquid with a reducing agent whereby vanadium impurities are converted into solid compounds, (d) separating pure TiCl₄ from the solid vanadium containing reaction products, and (e) separating the solid by-products and reaction products from TiCl₄ is improved by adding solid products of the vanadium reduction reactions according to step (c) to the crude TiCl₄ reaction product prior to the separating step (b).
 2. The process according to claim 1 wherein the separating in step (b) is by filtration.
 3. The process according to claim 1 wherein the separating in step (d) is by filtration and the TiCl₄ is then purified by distillation.
 4. The process according to claim 1 wherein TiCl₄ is separated from solid vanadium compounds in step (d) by distilling off TiCl₄.
 5. The process according to claim 4 wherein the solid vanadium containing reaction products are added to the crude TiCl₄ reaction product as the distilland from step (d).
 6. The process according to claim 4 wherein solids-containing distilland is filtered and the solids are added to the crude TiCl₄ reaction product.
 7. The process according to claim 1 wherein the reducing agent is an organic amine.
 8. The process according to claim 1 wherein the reducing agent is diphenyl amine.
 9. The process according to claim 8 wherein the amount of diphenyl amine is 2.5-5 kilograms per kilogram of vanadium.
 10. The process according to claim 1 wherein the reduction reaction of step (c) is at a temperature from 80°-125° C.
 11. The process according to claim 1 wherein the crude TiCl₄ reaction product is dechlorinated prior to the addition of reduced vanadium compounds according to step (e). 